A couple of interesting things I have learned as I went to change an existing booking (I class both ways):
(1) If you have purchased the waiver, you can make a change up to 8 days prior to departure. I was calling to see what a post-departure change to the return would cost and was told "hey, why dont you do it now, it would be free"
(2) the free predeparture change isn't as helpful as it sounds. It turns out that if you do that, it appears to refare the outbound to what is currently available to AAvacations. In my case, I is no longer available on the outbound (in fact only A is), so it raised the price quite a bit and I didnt make the change.
(3) the change to the return that i was looking to make was available in "I" (I6 according to EF) but the agent said "you should make up your mind, there is only one seat in I." That leads me to believe that the "I" bucket these folks see is a subset of what "I" values really are through other channels (like AA.com).
(4) It's $350 to make a change once outbound traveled. What I am interested in is whether once that is done, will I be able to call AA directly (not AAvacations) and have access to the full "I" bucket that I can see via EF?

I may fly the outbound and give this a try, I have some backups handy in the event that one of the many, many routings BRU-SEA that show "I" available both on AAvacations and EF are not available once I have departed.